1. Field of the Invention
The present invention relates to an inline degassing apparatus used for continuous degassing of nonferrous metal such as aluminum alloys and magnesium alloys.
2. Description of Related Art
During refining process of nonferrous metals such as aluminum alloys and magnesium alloys, a situation is frequently occurred that nonmetallic inclusions such as oxides are generated and hydrogen gas is mixed with the molten metal. Accordingly, a high quality after processing or working can only be achieved when a separating or removal of nonmetallic inclusions from the molten metal is done prior to the processing or working. Furthermore, by an introduction of molten metal containing solid dissolved gases including mainly hydrogen gas into a mold, small cavities called “pinholes” are likely generated after the solidification, resulting in a reduction in a degree of the compactness of the finished products. Furthermore, the existence of the inclusions attached to the gases may generate various defects in the product after subjected to a processing or working.
In view of the above, during the execution of a casting process of nonferrous metal such as aluminum alloy or magnesium alloy, a molten material is subjected to a degassing operation for increasing a quality of the molten metal prior to the execution of a casting operation. In such a degassing operation, a large quantity of finely bubbled inert gas such as argon gas or nitrogen gas is blown into the molten metal, so that solid dissolved gas and nonmetallic inclusions are entrapped or caught by the bubbles of the inert gas, which are floated for the removal.
FIG. 1 schematically illustrates a conventional structure of a degassing apparatus, which has been used for a continuous casting. The apparatus is placed between a holding furnace and a casting machine along a molten metal treatment line. The degassing apparatus receives molten metal 9 continuously through an inlet 2. The upper opening of a degassing container 1 is covered by a lid 3 and, at the downstream side, a partition 4 extends downwardly in the direction so that it crosses the flow of the metal 9 for preventing floating substances (suspended matter) including oxides etc., which is called as dross, from being flown into the subsequent treatment process. Namely, the partition 4 extends downwardly, so that a relatively narrowed passageway of a predetermined flow area is formed between the bottom end of the partition 4 and the inner bottom wall of the container 1. Such an arrangement of the partition 4 can obtain a maximized residence time of molten metal at the treating chamber 8 upstream from the partition 4, so that a prolonged duration of time of a degassing operation can be achieved. A rotary gas-diffusing device 5 is inserted through an aperture made in the lid 3 and is located in the molten metal in the degassing container 1. The gas-diffusing device 5 has a lower part located (immersed) in the molten metal while being subjected to a rotating movement, so that the inert gas is ejected from the lower part of the gas-diffusing device 5, while a finely bubbled inert gas is diffused into the molten metal.
A diffusion of an inert gas from the gas diffusing device 5 may cause a temperature the molten metal 9 to be dropped. Thus, it is quite likely that desired casting temperature cannot be maintained and in the worst case a solidification of the molten metal may be commenced. As a countermeasure, the degassing container 1 is provided with the burner 6 for generating a flame, which is directed through the aperture made in the lid 3 toward the molten metal in the container to keep the constant temperature.
Apart from the matter of degassing as discussed above, the burner 6 for heating of the metal in the container is also required to cause the metal remained in the container to be heated. Otherwise, a solidification of the metal remained in the container is started, which make it difficult that the metal remained in the container is smoothly molten together with the newly introduced metal into the container. Furthermore, when maintenance work is necessary in the degassing container, an operation of the burner 6 is essential for removing the molten metal remained in the container.
However, the aforementioned burner 6 heats the molten metal 9 from the upper side and, therefore, a difficulty is inevitably encountered that a heat cannot be easy reached to the molten metal in the lower position of the degassing container 1. In addition, this system is disadvantageous in that the flame of the burner promotes oxidation of the molten metal and an increased amount of the dross is generated.
To avoid such a problem, a patent convention treaty (PCT) publication WO95/13402 discloses an immersion type heater. This immersion type heater is inserted into the container through an aperture made in the lid of the container and its lower heating section is immersed or located in molten metal. Suppose as a construction of '402 patent that, instead of the burner shown 6 in FIG. 4, a longitudinally elongated cylindrical heater is arranged vertically along with sidewall of the container. The immersion type heater of '402 patent has an advantage that molten metal temperature goes up rapidly because, in comparison with the aforementioned burner heating system in FIG. 1, heat convection occurs easily owing to heating from the bottom.
The immersion type heater of '402 patent produces less amount of the dross compared with the burner heating system in FIG. 4. However, an amount of the dross, which is at any means not small amount, is still generated, which is largely attached to the portion of the heater corresponding to a location around the liquid-gas boundary in the container. By such an attachment of the dross, a removal of the heater through the heater insertion aperture at the rid becomes to be difficult. In such a situation, the heater together with the rid must be lifted, which is followed by a removal of the dross attached to the heater by scraping it. However, such a removal of the dross causes the heater to be instantly subjected to an outside air of low temperature, resulting in a rapid drop in a local temperature at a portion of the heater corresponding to a location around the liquid-gas boundary in the container. As a result, a highly increased thermal stress is generated in the heater, which frequently causes an outer protection tube to be damaged, which is made of relatively expensive ceramic material. In addition, '402 patent is also disadvantageous in an increased labor cost, which is needed for scraping the dross.
Furthermore, in '402 patent, an increased pressure is generated in the surface of the heater protection tube due to a swirl movement of the molten metal as generated by the diffusing operation of the rotary gas-diffusing device. Thus, a damage is likely generated not only in the heater protection tube but also in the heater assembly itself.
Furthermore, in case of the degassing apparatus of the '402 patent, the immersion type heater is arranged vertically close to the sidewall of the container. This is essential in the structure of the '402, in which the rotary gas-diffusing device occupies a substantial entire region of the center part of the available space inside the degassing apparatus. In this structure, the heater protection tube is inevitably subjected to great stress due to the swirl movement of the molten metal as generated by the operation of the rotary gas-diffusing device, resulting in a shortened service life of the heater protection tube, which makes the maintenance cost to be expensive. Furthermore, a non-uniformity in the temperature inside the apparatus is likely generated, which is disadvantageous not only from the view point of temperature control precision but also from the view point of thermal efficiency.